Semiconductor device technology requires accurate methods to characterize the semiconductor film and contact resistances. A transmission line model (TLM) test structure is conventionally used to extract these parameters. FIG. 1 shows a schematic of a TLM test structure 2 according to the prior art. The TLM test structure 2 includes a defined area of a semiconductor film 4 subject to characterization with a number of identical but differently spaced ohmic contacts, such as ohmic contacts 6A, 6B, formed on a surface of the semiconductor film 4. The TLM test structure 2 requires low-resistive highly-linear ohmic contacts 6A, 6B to extract accurate material parameters. One challenge to the development, characterization and optimization of wide bandgap heterostructures, such as high-Aluminum (Al) content AlGaN/GaN, is the formation of ohmic contacts 6A, 6B. In particular, the high temperatures required during annealing lead to substantial structural damage, increased roughness and changes the surface of the heterostructure, resulting in reduced sheet carrier concentration and mobility.
Capacitively-coupled contacts do not require annealing. Capacitively-coupled contact electrodes have very low contact impedance when the operating frequency is sufficiently high. In high frequency operations, capacitively-coupled contacts have been used in place of ohmic contacts.